1. Field of the Invention
The present invention relates to the field of optimization of the development of underground reservoirs such as hydrocarbon reservoirs, notably those comprising a fracture network.
2. Description of the Prior Art
The petroleum industry, and more precisely petroleum reservoir exploration and development, requires knowledge of the underground geology as perfectly as possible to efficiently provide evaluation of reserves, production modelling or development management. In fact, determining the location of a production well or of an injection well, the drilling mud composition, the completion characteristics, the parameters required for optimum hydrocarbon recovery (such as injection pressure, production flow rate, etc.) requires good knowledge of the reservoir. Reservoir knowledge means knowledge of the petrophysical properties of the subsoil at any point in space.
The petroleum industry has therefore combined for a long time technical measurements with modelling performed in the laboratory and/or by softwares. Petroleum reservoir modelling thus is an essential technical stage with a view to reservoir exploration or development. The goal of modelling is to provide a description of the reservoir.
Engineers in charge of the development of fractured reservoirs need to perfectly know the role of fractures. What is referred to as fracture is a plane discontinuity of very small thickness in relation to the extent thereof, representing a rupture plane of a rock of the reservoir.
On the one hand, knowledge of the distribution and of the behavior of these fractures allows optimizing the location and the spacing between wells to be drilled through the oil-bearing reservoir.
On the other hand, the geometry of the fracture network conditions the fluid displacement, at the reservoir scale as well as the local scale where it determines elementary matrix blocks in which the oil is trapped. Knowing the distribution of the fractures is therefore also very helpful, at this stage, to the reservoir engineer who wants to calibrate the models he or she constructs to simulate the reservoirs in order to reproduce or to predict the past or future production curves.
Engineers in charge of the development of fractured reservoirs therefore need to estimate the large-scale permeability (scale of the drainage radius of a well or of the interwell space for example) of the fracture networks and to forecast the hydrodynamic behavior (flow rate, pressure, etc.) of these networks in response to exterior stresses imposed via wells.
Geosciences specialists therefore first carry out characterization of the fracture network in form of a set of fracture families characterized by geometrical attributes.
Then, with a view to simulation of the flows within the fractured reservoir, a numerical model is used most often. This model is applied to a discretized representation of the reservoir, that is the reservoir is divided into a set of grid cells. Application of the numerical model requires knowledge of the flow properties of the fracture network at the cell scale, usually of hectometric size. In particular, the permeabilities of the fracture network have to be determined.
This can be reliably achieved from a flow calculation carried out on a geometrical model representative of the fracture network. Such a method is described in French Patent 2,757,947 and corresponding U.S. Pat. No. 6,023,656.
However, this numerical calculation method is costly in calculating time for complex and/or large-size reservoirs. Discretization of a reservoir often leads to the construction of a grid comprising millions of cells.
The specialist then has alternative methods available, in fact analytical calculation methods. What is referred to as analytical method is one or more equations allowing precise determination, without approximations or numerical (iterative, etc.) techniques, the unknowns of a problem according to the data. An analytical method example is for instance described in the following document:    M. Chen, M. Bai and J.-C. Roegiers, Permeability Tensors of Anisotropic Fracture Networks, Mathematical Geology, Vol. 31, No. 4, 1999.
However, analytical methods are most often based on hypotheses simplifying the physical problem and these methods do not allow obtaining the accuracy reached by the numerical methods that allow the real complexity of physics to be fully taken into account. It is however sometimes crucial to preserve a high accuracy in the permeability estimation of fracture networks so as to be able to select the best production scenarios allowing the hydrocarbon production to be optimized.
The invention is a method for optimizing the development of a hydrocarbon reservoir comprising a fracture network, wherein the network permeability is determined by means of a reliable compromise between numerical and analytical methods.
The method achieves this by carrying out a quantitative analysis of the connectivity properties of the fracture network, so as to limit the use of numerical methods.